Insulated-Gate Bipolar Transistors (IGBTs) are three-terminal power semiconductor devices that combine the gate-drive characteristics of a Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) with the high-current and low-saturation-voltage capability of a bipolar transistor. Modern IGBT devices are formed by integrating a FET and a bipolar power transistor on the same silicon die. The FET functions as a control input while the bipolar power transistor is used as a switch. IGBTs efficiently switch electric power in many applications such as electric motors, variable speed refrigerators, air-conditioners, etc. However, these applications have considerably high inductive loads which can cause current to flow in a reverse direction of the switch. If this reverse current is commutated into the IGBT, the device will be destroyed. Therefore, diodes can be used to conduct this reverse current and thereby protect the IGBT.
One technique to enable the IGBT to conduct the reverse current is to integrate a freewheeling diode into the IGBT device. The collector electrode of the IGBT is divided into different regions of n and p-type material. The p-type regions form the IGBT collector. The n-type regions, in conjunction with the n-type drift zone of the IGBT device, form a freewheeling diode with the p-body and a heavily doped p-type anti-latchup region of the IGBT device.
Integrating a freewheeling diode with an IGBT device creates some problematic conditions. Mainly, power continues to dissipate in a freewheeling diode in conduction mode and even after it has been reverse biased. Current will continue to flow until the diode reaches a steady-state reverse bias condition. The condition when the diode changes from forward conduction to blocking is commonly referred to as reverse recovery. The Reverse Recovery Charge (RRC) causes the integrated freewheeling diode to incur electrical losses. These electrical losses increase when the diode is integrated in the IGBT. Some applications cannot tolerate the resulting elevated temperature and/or power conditions. In addition, the elevated temperature and power consumption reduces the lifetime of the IGBT.
Electrical losses caused by integrating a freewheeling diode with an IGBT device can be lowered by reducing the RRC of the diode. Diode RRC can be lowered by reducing the concentration of free-charge carriers within the IGBT device in diode mode. Most free-charge carriers originate within the IGBT device from the highly doped anti-latchup p-type region of the device. This highly doped region injects free-charge carriers into the drift zone of the IGBT device in diode mode. Accordingly, the diode RCC can be reduced by lowering the doping concentration of the highly doped anti-latchup p-type region. However, this significantly reduces the latch-up robustness of the IGBT device and is not a practical solution for most IGBT applications because IGBT performance degrades.
Some conventional approaches involve forming a single or local reduced charge-carrier lifetime region in the drift zone of the IGBT device. This single region must have a very low charge carrier lifetime to sufficiently reduce the RCC of the freewheeling diode integrated with the IGBT device. A single reduced charge-carrier lifetime region is typically formed by irradiating either the front or back side of the wafer on which the IGBT device and freewheeling diode are fabricated. The irradiation treatment may result in two zones being formed within the single reduced charge-carrier lifetime region. One zone has a charge carrier lifetime that is higher than that of the second zone, but lower than that of the non-irradiated part of the IGBT drift zone. However, the single region must still have a very low charge carrier lifetime to be effective at reducing diode RRC. Forming a very low charge carrier lifetime region in the drift zone of an IGBT increases the VCESat (collector-to-emitter saturation voltage) of the IGBT and also leakage current during blocking mode. Moreover, the circuit designer must still trade-off between high emitter efficiency and low diode RRC. Forming a single reduced charge-carrier lifetime region conventionally yields a stored charge that is at least three times higher than that of a single non-freewheeling diode.